To obtain a wide band antenna design in an exemplary embodiment of the present invention, a relatively constant pattern and impedance over the desired frequency range may be achieved. Both of these aspects are essentially dependent on the antenna current, which implies a relatively constant current distribution over the desired frequency range. Generally, there are two underlying design goals in an exemplary embodiment of the present invention. The first goal is to preserve a relatively constant current distribution along the antenna over the desired frequency range to achieve broad bandwidth in terms of pattern. The second goal is to keep the current magnitude and phase at the feeding port nearly constant over the frequency band to achieve a wide input impedance bandwidth. Broadband behavior may therefore be obtained by shaping the antenna current distribution over frequency, using several techniques.
Broadband antenna design may comprise two aspects: relatively constant pattern and impedance over frequency. Both of these aspects are essentially dependent on the antenna current, which implies a relatively constant current distribution over frequency. Broadband behavior may therefore be obtained by shaping this current distribution over frequency, using several techniques. The theory of characteristic modes allows for the analysis and synthesis of antenna currents.[X][I]=λ[R][I]
Known work involving characteristic modes has also involved pattern synthesis. In the known art, a method is provided by which any real current could be resonated given that the antenna was properly loaded with reactive elements. Thus at single frequencies, an antenna could be made to have an arbitrary pattern, provided that the current distribution which generated such a pattern is known. Also, the known art touched on the problem of bandwidth. However, the frequency behavior of the loads has not been discussed yet in the known art. Current research by the present inventors will show that the practical implementation of the loads over a wide frequency bank requires special consideration.
In order to apply characteristic mode theory to the development of broadband wire antennas, such frequency behavior must first be understood and the implications need to be considered. Therefore, simple wire antennas were analyzed using the Method of Moments. Then characteristic mode theory was applied over multiple frequencies to synthesize desired current distributions. In exemplary embodiments of the present invention, antennas show broadband behavior in both impedance and pattern.
Exemplary embodiments of the present invention include methods for designing an antenna as well as the resulting antenna. In one example, a method for designing an antenna is comprised of the following steps: 1) determining a desired current distribution over an antenna; 2) determining a number and location of at least one port over the antenna; 3) determining at least one desired load to achieve the desired current distribution; 4) providing at least one desired load with at least one lumped Foster or non-Foster circuit elements; and 5) determining current and radiation patterns over a desired frequency band. Other exemplary embodiments of the present invention may include one or more of such steps or various combinations or orders of such steps, which will be evident based on the present specification.
In the foregoing example, the step of determining the desired current distribution may be based on a desired radiation pattern and input impedance. For example, the step of determining the desired current distribution may enable a substantially constant radiation pattern and input impedance over a wider frequency band in comparison to a conventional electrically small to mid-size antenna. As further examples: 1) the step of determining the number and location of at least one port over the antenna may comprise determining the number and location of at least one port to sufficiently control the desired current distribution; 2) the step of determining at least one desired load to achieve the desired current distribution may comprise using Characteristic Mode Theory to compute at least one desired load sufficient to resonate the desired current distribution at least one port over the desired frequency band; 3) the step of providing at least one desired load with at least one lumped Foster or non-Foster circuit elements may comprise providing at least one desired load with at least one lumped Foster or non-Foster circuit elements over the desired frequency band; and 4) the step of determining the current and the radiation patterns over the desired frequency band may comprise determining input impedance over the desired frequency band. In addition, an exemplary method may further comprise the step of modifying the number and the location of at least one load until the desired current and radiation patterns are achieved. For example, the step of modifying the number and the location of at least one load until the desired current and radiation patterns are achieved may comprise the steps of adjusting the number and the location of at least one port and repeating the following steps until the desired current and radiation patterns are achieved: 1) determining at least one desired load to achieve the desired current distribution; 2) providing at least one load with at least one lumped Foster or non-Foster circuit elements; and 3) determining the current and radiation patterns over the desired frequency band.
In another exemplary embodiment of the present invention, an antenna may comprise at least one port; and at least one desired load with at least one lumped Foster or non-Foster circuit elements. In such an embodiment, the antenna may be adapted to provide a substantially constant radiation pattern and input impedance over a wide frequency band. An example of such antenna may be a wideband (e.g., ultrawideband) antenna. Furthermore, an exemplary embodiment of such antenna may be in a size range of electrically small to mid-size. Other variations may be possible.
Exemplary embodiments of the present invention may provide or enable various advantages or benefits. For instance, exemplary embodiments of the present invention may offer two innovations, namely, a method to design wideband antennas and the use of non-Foster components to load an antenna. In an exemplary embodiment, these components may provide additional degrees of freedom not available with known passive capacitors, inductors, and resistors. For example, in an exemplary embodiment of the present invention, the theory of Characteristic Mode (CM) may be used to load electrically small to mid-size antennas with non-Foster elements (e.g., negative valued capacitors and inductors) to force an antenna to preserve a fairly constant radiation pattern and input impedance over a wider frequency band. Furthermore, such loading may lower the Q factor of the antenna allowing a much higher bandwidth of operation than what conventional antennas can achieve if complemented with a passive matching network. Unlike conventional methods, an example of this method may allow for controlling both the pattern and impedance bandwidth loading of the antenna structure with Non-Foster components. The design of these loads may be done with the Method of Characteristic Modes. In contrast to exemplary embodiments of the present invention, other loading techniques of the known art do not generally work for electrically small to mid-size antennas. In particular, most wideband antennas are designed by choosing a geometry for the antenna and by adding dielectric, magnetic, or other exotic materials that usually have some loss.
Various other benefits or advantages of exemplary embodiments of the present invention may include one or more of the following: 1) easy retrofit with existing infrastructure, as added active loads may work with existing antennas; 2) using loads, the antenna current shape (and pattern shape) may be controlled over a desired band; 3) loaded antennas may be operated with less complex matching networks; 4) may be utilized in building an integrated antenna (e.g., on chip Technology—in other words, compatible with VLSI technology); and 5) may be no need to use exotic materials with hard to obtain electrical properties to achieve UWB antennas.
As a result of one or more of the aforementioned benefits, applications may include, but are not limited to, any UWB small antennas—including antennas for commercial, medical, homeland security, RFID, and other applications where electrically small integrated antennas may be useful or required. Other suitable applications include, but are not limited to, applications where wideband on-chip antennas are useful or required.
In addition to the novel features and advantages mentioned above, other benefits will be readily apparent from the following descriptions of the drawings and exemplary embodiments.